1. Field of the Invention
The present invention relates to a method of making a blank in an apparatus for forming an image onto a recording medium.
2. Related Background Art
As an example of such a kind of image recording apparatus, a laser beam printer (hereinafter, abbreviated as an LBP) will be described. FIG. 16 shows an example of conventional LBP apparatuses disclosed in U.S. Pat. Nos. 4,059,833, 4,175,851, and the like. Reference numeral 1 denotes a laser driving circuit; 2 indicates a laser beam emitting device; 3 a polygonal mirror; 4 a photo sensitive drum; 5 a primary charger; 6 a developing device; 11 a copy transfer paper; 13 a conveying roller; 14 a registration roller; 15 a copy transfer charger; 16 a separating charger; 17 a cleaner; 20 a cassette; and 21 a paper feed roller.
An image signal which is given from a host computer is transmitted through the laser driving circuit 1 and actuates the laser beam emitting device 2. A laser beam emitted from the laser beam emitting device 2 scans on the surface of the photo sensitive drum 4 by an optical system (not shown) and polygonal mirror 3, so that a latent image is formed onto the drum 4. The toner is transferred onto the surface of the drum 4 in accordance with the latent image and is developed by the developing device 6. The developed image on the drum 4 is transferred onto the copy transfer paper 11 by the copy transfer charger 15. The image transferred paper 11 is then separated from the drum 4 by the separating charger 16. Thereafter, the surface of the drum 4 is cleaned by the cleaner 17 for preparation in the next use. The paper 11 is picked up from the cassette 20 by the paper feed roller 21 and is conveyed by the conveying roller 13 and is fed to the registration roller 14 section. In the inoperative mode, the registration roller 14 abuts on the front edge of the paper 11 and is driven at a proper timing when the front edge of the toner image on the drum 4 coincides with the front edge of the copy transfer paper, thereby preventing that the positional relation between the toner image on the drum 4 and the paper 11 is deviated.
However, the position of the toner image on the drum 4, the position of the front edge of the paper 11, and the like, that is, the relative positional relation between the toner image on the drum 4 and the paper 11 cannot help fluctuating within a predetermined range due to errors of the attaching positions of various parts constructing the apparatus and errors of the generating timings of control signals which are caused due to an error of a reference clock frequency.
Therefore, when an image signal from the host computer is intended to be fully recorded onto the whole paper, the toner corresponding to the peripheral edge portion of the paper of the toner image on the drum 4 is not transferred but remains on the drum 4. Therefore, the untransferred toner is dispersed into the apparatus and the primary charger 5, copy transfer charger 15, separating charger 16, and the like are polluted. Thus, when the apparatus is used for a long time, there occurs a problem on quality such as image variation, stripe, or the like.
On the other hand, there is also a case where the copy transfer paper is hard to be separated from the drum and the paper jam occurs.
To prevent such problems, in general, for the image signal from the host computer, the signal corresponding to the presumed peripheral edge portion of the paper is forcedly cut on the LBP side and a blank is formed.
FIG. 17 shows an example of a timing chart to form a blank in the conventional apparatus.
A front edge signal of the image signal which is given from the host computer is transferred to the laser beam emitting device 2 through the laser driving circuit 1 after an elapse of .tau..sub.0 seconds from a laser signal reference such as vertical sync signal VSYNC indicative of the start of one screen, horizontal sync signal HSYNC indicative of the start of one line, or the like. The signal from the front edge signal of the image signal to the back edge signal after an elapse of .tau..sub.i seconds is written onto the drum 4.
As the horizontal sync signal HSYNC, a beam detecting signal BD which is generated when the reference position of the laser beam which is scanned is detected is used.
A blank is formed by cutting the signal by only the amount of .tau..sub.b seconds in response to the front and back edges of the image signal. That is, at the front edge of the image signal, the signal in a range from .tau..sub.0 seconds after the laser signal reference to (.tau..sub.0 +.tau..sub.b) seconds is cut. At the back edge, the signal in a range from (.tau..sub.0 +.tau..sub.i -.tau..sub.b) seconds after the laser signal reference to (.tau..sub.0 +.tau..sub.i) seconds is cut.
After an elapse of .tau..sub.1 seconds from the laser signal reference, the driving of the registration roller 14 is started so that the front edge of the image signal coincides with the front edge of the paper 11. The value of .tau..sub.1 is calculated by the following equation. ##EQU1## where, l.sub.1 (mm): distance from the laser writing position on the photo sensitive drum 4 to the copy transfer position,
l.sub.0 (mm): distance from the registration roller 14 to the copy transfer position, PA1 v (mm/sec): peripheral velocity of the drum 4.
FIG. 16 shows an example of l.sub.0 &lt;l.sub.1. However, when l.sub.1 &lt;l.sub.0, there is a relation of .tau..sub.1 &lt;.tau..sub.0. In the formation of the blank in the foregoing conventional example, the front and back edges of the image signal have been cut by only the same length .tau..sub.b.
FIG. 18 is a diagram showing a state in which the blanks and image signal formed as mentioned above are shown on presumed paper.
That is, FIG. 18 shows the image signal obtained by synthesizing the image signal given from the host computer and the blank signal forcedly formed on the LBP side by giving the concept of a bit map. Blanks A, B, C, and D are formed in correspondence to the front, back, left, and right edges of the presumed paper, respectively. W denotes a paper width in the main scan direction of the laser beam and L indicates a paper length in the subscan direction of the laser beam or in the rotating direction of the drum 4.
The dimensions of the blanks are set such that A (mm)=B (mm) or C (mm)=D (mm) in correspondence to the blank signal .tau..sub.b.
A practical example will now be described hereinbelow. In the example, the blanks having the dimensions of A=B=C=D=2.5 (mm) are formed. Actually, since there is an error of oscillating frequency of the quartz oscillator as a reference of a clock signal on signal control, the values of the blanks have errors. The blank errors are obtained by multiplying a predetermined ratio to the time (distance) (t.sub.0 seconds in the case of the front edge A and (.tau..sub.0 +.tau..sub.i -.tau..sub.b) seconds in the case of the back edge B) from the laser signal reference to each blank signal on the timing chart of FIG. 16. However, in general, since .tau..sub.0 &lt;&lt;.tau..sub.i and .tau..sub.b &lt;&lt;.tau..sub.i, the error of the blank in the edge portion in which the distance from the laser signal reference is larger increases more.
Table 1 shows an example in the case where the paper size is set to the A4 size (W=210 mm, L=297 mm), the distance between the laser signal reference VSYNC and the front edge of the image is set to 5 mm, the distance between the laser signal reference HSYNC and the left edge of the image is set to 10 mm, the reference blank amount is set to 3.5 mm, and the error of the control signal is set to .+-.0.2%.
TABLE 1 ______________________________________ (unit: mm) A B C D ______________________________________ Blanks on 3.5 .+-. 0.1 3.5 .+-. 0.6 3.5 .+-. 0.1 3.5 .+-. 0.5 image signal ______________________________________
On the other hand, the blank width on the final print is influenced by the factors of errors from the ideal image position (a magnification error which is caused by the optical system when a latent image is formed by the laser, a registration error and oblique movement amount regarding the conveyance of the paper, and the like) and by dimension errors of the paper itself, so that it will be obvious from Table 2 that a side having no blank exists on the paper with a certain probability.
TABLE 2 ______________________________________ (unit: mm) Synthe- Tolerance concerned sized Width Obli- value of Magni- que Size of blank fica- Regis- Move- of tole- on tion tration ment paper rances print ______________________________________ A -- 0 .+-. 2.0 0 .+-. 0.7 -- 0 .+-. 2.2 3.5 .+-. 2.3 B 0 .+-. 3.0 0 .+-. 2.0 0 .+-. 0.7 0 .+-. 0 .+-. 3.8 3.5 .+-. 4.4 1.0 C -- 0 .+-. 2.0 0 .+-. 1.0 -- 0 .+-. 2.3 3.5 .+-. 2.4 D 0 .+-. 2.1 0 .+-. 2.0 0 .+-. 1.0 0 .+-. 0 .+-. 3.3 3.5 .+-. 3.8 1.0 ______________________________________
The actual variations of the tolerances have a normal distribution. The overlap of the various tolerances is not given by the simple sum but is given by the square of the sum of the values which are obtained by squaring each tolerance. On the other hand, the reason why the "magnification" and "paper size" of the sides A and C have no tolerance is because those sides are used as reference sides to measure the tolerances. That is, all of the "magnification", "registration", and "oblique movement" in Table 2 are the amounts which are measured by the image on the final print and the specified values are predetermined to limit those amounts. The "magnification" is measured by using the paper edge (left edge C in the example) on the reference side in the main scan direction of the laser beam and the paper edge (edge A in the example) on the reference side in the subscan direction as references. Since the paper is fed on the basis of the front edge A and either the right or left paper edge (left edge C in the example) as references, the errors are accumulated at the paper edges on the opposite sides, respectively. Therefore, in general, the synthesized values of the tolerances at the front edge A and back edge B or at the left edge C and right edge D differ.
However, since the blanks on the image signal are formed such that A=B and C=D, there are the following inconveniences.
(1) In certain paper edge portions (front edge A and left edge C in the example), the signal is cut more than it is needed, thereby narrowing the signal region which can be effectively used.
(2) On the contrary, in the other paper edge portions (back edge B and right edge D in the example), the blanks lack and there occurs a case where the toner image is not transferred but remains onto the photo sensitive drum 4.
In the case of using the LBP as a conventional line printer, in general documents, blanks are frequently formed in the peripheral edge portions of the paper. Therefore, such inconveniences are not so serious. However, in the case of handling image information as in the case of using the LBP as an output printer of an electronic file or image reader, or the like, it is demanded to write information on the whole region of the paper. The signal region which can be effectively used must be widened as large as possible and a consideration must be paid such that no toner image remains on the photo sensitive drum 4. Such considerations lack in the conventional blank formation.
On the other hand, in U.S. patent application No. 779,107 and the like, there has been disclosed an image recording apparatus having the functions to print on both sides of a copy transfer paper and to print on a single side of a copy transfer paper a plurality of times. In such an apparatus, the running accuracies of the papers differ in the printing operations of the first and second surfaces due to the difference of the paths of the paper or the like. Therefore, there is a case such that although blanks are formed on the first surface, no blank is formed on the second surface, so that the image is deviated out of the paper.
Particularly, in the case of transferring by using the copy transfer roller, there is a drawback such that the toner remaining on the drum without being transferred is deposited onto the copy transfer roller, so that the back surface of the next copy transfer paper becomes dirty or the like.